1. Technical Field
The present invention relates generally to a power control system and method; and more particularly to a power control system for providing the output power control of a generator by shifting power curves of the generator.
2. Description of Related Art
The common solutions for the problem that the wind power generation system is operated at the over-rated wind speed condition can be divided into the variable pitch control, yaw control, load-dumping control, torque increasing control, and direct short-circuit control. The variable pitch control is used to indirectly capture wind energy by controlling the windward angle of the blade. Also, the variable pitch control is usually realized for the middle-scale and large-scale wind power generation system because of high costs, high damage rate, and complicated control of the blade mechanism. The yaw control is used for reduction of capturing wind energy by tail deviation. However, the joints of the tail are easily damaged when the tail swings for months and years. The load-dumping control is that a load is used at the output side of the generator to consume superfluous energy. Because a third-order relationship is between the output power of the generator and the wind speed, a large number of loads are needed once the generator is operated at the over-rated wind speed. The torque increasing control is used to short unnecessary coils except the loading coil inside the generator so that the torque is increased to resist the blade so as to reduce the rotation speed and the output power. The direct short-circuit control is used to directly short the generator to stop operating at the rated wind speed. However, unstable wind conditions probably make the generator fail in loading due to instantaneous gusty wind, thus affecting overall power generation.
FIG. 1 is a circuit diagram of a load-dumping system of a conventional wind turbine. The load-dumping resistors 34 are used to consume electricity power generated from the generator. In particular, resistances of the load-dumping resistors 34 are substantially equal to loadings applied from an electric grid 26 to the generator. The load-dumping resistor 34 shown in FIG. 1 is the rated resistance of each phase of the three-phase power source of a permanent magnet generator (PMG) 36. In addition, the permanent magnet generator 36 is driven by a wind turbine.
A programmable logic circuit (PLC) 42 is a controller of the wind turbine to monitor the connection between the electric grid 26 and the permanent magnet generator 36, and detect losses of the load at a side of the electric grid 26. When the losses of the load are detected, a thyristor bridge circuit 44 is turned on by the programmable logic circuit 42 so as to connect the load-dumping resistors 34 to each phase of the output power of the permanent magnet generator 36, respectively. Accordingly, the load-dumping resistors 34 are used to consume electricity power generated from the generator until the load and the converter are re-connected to reduce blade speed of the wind turbine.